This invention relates to phased array semiconductor lasers having multi-emission or broad emission capabilities and in particular to a phased array laser or phased locked array laser having preferred fundamental supermode operation with a structural design that utilizes impurity induced disordering (IID).
Phased array semiconductor lasers comprise a plurality of closely coupled or spaced emitters on the same integral structure or substrate. Examples of such phased array lasers are illustrated in U.S. Pat. No. 4,255,717, now Pat. No. Re. 31,806, and in an article of William Streifer et al., entitled "Phased Array Diode Lasers", published in the June 1984 Issue of Laser Focus/Electro-Optics. The emitters of such a laser are represented by the periodically spaced current confinement means, e.g. stripes, for current pumping and establishment of spaced optical cavities in the active region of the structure. The current confinement means may be interconnected or closely spaced to a degree that the optical mode established in each of the lasing cavities below a respective current confinement means couples to its neighboring optical modes, i.e., the evanescent wave overlaps into adjacent optical lasing cavities. The array of optical fields produced become locked in phase, and, if the phase difference between adjacent current confinement means is zero, the lateral radiation pattern in the far field will comprise a single lobe. However, as explained in the above-mentioned article, the phased array laser does not operate in a single mode but rather generally operate with two or more lobes in the far field pattern. The phase relationship between adjacent optical modes is not under independent control and the phases will adjust themselves in a manner toward minimizing laser threshold current. In most cases, it appears that the lasing mode favored is a supermode wherein the optical field between adjacent optical emitters passes through zero. This is because in most real refractive index lasers as well as many gain guided lasers, pumping is reduced at locations between the laser emitters requiring overall reduced current pumping.
The foregoing explanation can be exemplified as follows. An array laser with N coupled emitters has N possible coupled modes, which are referred to as "supermodes". A supermode is a cooperative lasing of the N optical emitters or filaments of the array laser. Since there are N emitters, there are N possible supermodes since all these emitters are optically coupled together.
Each supermode has the property that the 1.sup.st and the N.sup.th supermode have the same intensity pattern or envelope, the 2.sup.nd and the (N-1).sup.th have the same intensity envelope, and, in general, the i.sup.th and (N-i).sup.th have the same intensity envelopes. The 1.sup.st or fundamental supermode has all emitters lasing in phase with an amplitude distribution representative of half a sinusoidal cycle. This is the only supermode pattern that radiates in a single central lobe in the far field pattern because all emitters are in phase.
Thus, for a uniformly spaced array of identical emitters, the 1.sup.st and N.sup..sub. supermode envelopes are half a sinusoidal period, the second and the (N-1).sup.th supermode envelopes are two half sinusoidal periods, etc. The phase relationship between the individual emitters in N supermodes differ. Specifically, for the 1.sup.st supermode, all emitters are in phase and for the N.sup..sub. supermode, the phases alternate between zero and .pi.. Usually the 1.sup.st and N.sup.th supermodes have the lowest current thresholds as compared to all other supermodes because their intensity envelopes do not exhibit nulls near the center of the array where the charge density is greater as a result of current spreading and charge diffusion in the active region of the array laser. However, as previously indicated, the N.sup.th supermode, which radiates in two lobes, has a lower current threshold of operation than the 1.sup.st supermode.
Phased array lasers have high utility due to their high power output. It is preferred that the power be concentrated in a single lobe, i.e., in the 1.sup.st supermode. The reason is that a substantial majority of laser applications require power in a single far field lobe. If lasing is experienced in more than one lobe, measures are taken to diminish or otherwise attempt to eliminate or block off the other operating lobes in the far field pattern.
Recently, there has been much activity relative to phase locked array lasers or phased array lasers where efforts have been established to discriminate among the supermodes and provide fundamental supermode selection. One such suggestion was at the IEEE 9th Conference in Brazil, July, 1984 wherein J. Katz et al presented a talk on supermode discrimination by controlling lateral gain distribution along the plane of the lasing elements by incorporating a separate contact to each laser array element and tailoring the currents through the array laser elements. The abstract for the talk is found in the Proceedings of the Conference at pages 94 and 95 entitled "Supermode Discrimination in Phase-Locked Arrays of Semiconductor Laser Arrays".
More recently is the articles of Twu et al entitled "High Power Coupled Ridge Waveguide Semiconductor Laser Arrays", Applied Physics Letters, Vol. 45(7), pp. 709-711 (Oct. 1, 1984) and of S. Mukai et al entitled Fundamental Mode Oscillation of Buried Rdge Waveguide Laser Array", Applied Physics Letters, Vol. 45(8), pp. 834-835 (Oct. 15, 1984). These articles suggest discrimination among the supermodes to obtain the single lobe fundamental supermode by employing index guided ridge waveguide structure wherein the laser elements are uniformly pumped with the optical field mainly confined to the ridge region of the structure while higher gain is experienced in the valley or coupling regions to induce in-phase operation (0.degree. phase) and promotion of fundamental supermode operation. Further techniques to discriminate among supermodes are illustrated in U.S. patent application Ser. No. 667,251 filed Nov. 1, 1984, U.S. Pat. No. 4,624,000, entitled "Phased Array Semiconductor Lasers With Preferred Emission in a Single Lobe" and assigned to the same assignee herein. The techniques proposed in this application relate to the use of structural means associated with the laser to enhance the amount of gain experienced in regions between adjacent optical cavities of lasing elements by spatially modulating the optical overlap of the optical field of each of the laser elements across the array to thereby favor the fundamental supermode over other potential modes.
The present invention represents an improved phased array semiconductor laser providing the desired fundamental supermode operation but utilizing a more streamline structural configuration providing a much simpler method of fabrication by employing Impurity Induced Disordering (IID) techniques.